Active matrix display devices such as liquid crystal display devices and organic electroluminescent (EL) display devices use TFTs as switching elements or driver elements.
In recent years, active development has been conducted on, as next-generation TFTs, oxide semiconductor TFTs which use an oxide semiconductor such as InGaZnOx (IGZO) for channel layers. For example, Patent Literature (PTL) 1 describes an oxide semiconductor TFT having an oxide semiconductor layer as a channel layer.